1. Field of the invention
This invention relates to a desulfurization and decarbonation process in which a gas containing sulfur oxides and carbon dioxide is treated with a basic calcium substance and a basic amine compound absorbent.
2. Description of the related art
In recent years, a variety of exhaust gas treating processes for use in facilities discharging large quantities of exhaust gas have been proposed from the viewpoint of air pollution prevention and global environment cleaning. For example, thermal electric power plants and boiler equipment use large quantities of coal, heavy oil and superheavy oil as fuels, and involve problems concerning volume and concentration control of the emission of sulfur oxides (typically sulfur dioxide), nitrogen oxides, carbon dioxide and the like.
In particular, sulfur oxides may produce sulfuric acid mist and acid rain in the presence of atmospheric moisture and thereby cause damage to human bodies, crops, forest and the like. Consequently, dry and wet treating processes have conventionally been proposed and practiced. For example, in such facilities, processes using lime as absorbent to form gypsum are being predominantly employed from an economic point of view.
Moreover, from the viewpoint of global warming, the control of the emission of carbon dioxide, together with Freon gases and methane gas, is recently examined. To this end, various methods such as PSA (pressure swing method), membrane separation/concentration, fixation by reaction with basic compounds, fixation by the anabolic action of plants, liquefaction or solidification of separated and purified carbon dioxide, and fuel regeneration by hydrogenation are investigated.
As an example of the prior art, a process for effecting desulfurization and decarbonation at the same time is proposed in Japanese Patent Provisional Publication No. 6-86911/'94. This process permits simplification of the equipment, but is not satisfactory in that a large amount of an amine absorbent is required for the purpose of absorbing carbon dioxide present in large quantities in addition to sulfur oxides, and the complete liberation of carbon dioxide alone is not necessarily advisable from the viewpoint of equipment and energy requirements. Moreover, in some cases, carbon dioxide may not be completely removed, thus affecting the quality of gypsum recovered.
Furthermore, in this process, the amine sulfate formed in the absorption tower is decomposed with the aid of sodium hydroxide or the like to regenerate the amine. However, since a considerable amount of sodium sulfate formed as a by-product is dissolved in the liquid phase, it is necessary to evaporate the amine and thereby separate it from sodium sulfate. This is disadvantageous in view of the amount of heating energy required and the possible deterioration of the amine.